When no mesh config exists (fresh install), scan for serial devices
at /dev/ttyUSB* and /dev/ttyACM*. If a radio is found, auto-enable
mesh and save the config so subsequent boots connect immediately.
Previously, mesh defaulted to disabled and the radio was never probed
unless the user manually created a mesh-config.json file.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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